Crosslinkable cellulosic fibrous product

ABSTRACT

A crosslinkable cellulosic fibrous product that includes cellulosic fibers and a crosslinking agent is disclosed. The crosslinkable cellulosic fibrous product can be formed as a web or sheet that has structural integrity and sheet strength sufficient to permit the fibrous web to be rolled, transported, and used in rolled form in subsequent processes. The crosslinkable fibrous product can be converted into a crosslinked fibrous product by subjecting the product to conditions sufficient to effect intrafiber crosslinking. Alternatively, the web can be fiberized and the resulting individual crosslinkable fibers combined with other fibers and/or other materials to provide a fibrous web containing crosslinkable cellulosic fibers. Subjecting such a web to crosslinking conditions provides a fibrous web that includes, in addition to other fibers or materials, crosslinked cellulosic fibers.

FIELD OF THE INVENTION

The present invention relates generally to a cellulosic fibrous productand, more particularly, to a crosslinkable cellulosic fibrous productthat includes cellulosic fibers and a crosslinking agent.

BACKGROUND OF THE INVENTION

Crosslinked cellulosic fibers are advantageously incorporated into avariety of fibrous products to enhance product bulk and resilience.Absorbent articles, such as diapers, are typically formed from fibrouscomposites that include, in addition to crosslinked cellulosic fibers,absorbent fibers such as wood pulp fibers. When incorporated intoabsorbent articles, such fibrous composites can provide a product thatoffers the advantages of high liquid acquisition rate and high liquidwicking capacity imparted by the crosslinked fibers and the absorbentfibers, respectively. However, fibrous composites that includerelatively high percentages of crosslinked fibers suffer from low sheetstrength.

The relatively low strength of sheets that include crosslinked fibers isdue to the loss of hydrogen bonding sites that accompanies cellulosecrosslinking. As a result of their chemical modification, crosslinkedcellulosic fibers lack hydroxyl groups that are necessary for forminghydrogen bonds between fibers. The inability of crosslinked fibers toform interfiber bonds generally precludes their formation into sheets orwebs having any significant structural integrity. Thus, unlike othercellulosic fibrous materials that can be formed into sheets or webs andthen transported as rolled goods to a customer for further processing oruse, crosslinked fibers are generally transported as bales.

Furthermore, in contrast to other cellulosic fibrous products that canbe readily produced, transported, and used in processes as rolled goods,the handling and use of crosslinked cellulosic fibrous bales can bedifficult and costly. For example, bale opening equipment is expensive.In addition, fibrous bale handling often results in the creation ofdust, a potential health hazard.

Accordingly, there exists a need for a crosslinked cellulosic fibrousproduct that can be readily formed, transported, and used in subsequentprocesses. Alternatively, there is a need for a readily formed andtransported cellulosic fibrous product that can be converted to a usefulcrosslinked cellulosic fibrous product on arrival at the processingsite. Such a fibrous product is ideally converted to a crosslinkedcellulosic product without the need for expensive equipment and/oradditional time-consuming processing. The present invention seeks tofulfill these needs and provides further related advantages.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a crosslinkable cellulosicfibrous product that includes cellulosic fibers and a crosslinkingagent.

In another aspect of the invention, a method for forming a crosslinkablecellulosic fibrous product is provided.

In a further aspect, the present invention provides methods for usingthe crosslinkable cellulosic fibrous product.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The fibrous product of the present invention is a crosslinkablecellulosic fibrous product that includes cellulosic fibers and acrosslinking agent. The crosslinkable cellulosic fibrous product can beformed as a web or sheet that has structural integrity and sheetstrength sufficient to permit the fibrous web to be rolled, transported,and used in rolled form in subsequent processes. As used herein, theterm “crosslinkable” refers to a fibrous cellulosic web that has beentreated with a crosslinking agent, but has not been subjected toconditions sufficient to effect substantial cellulose crosslinking.

The crosslinkable product of the invention includes cellulosic fibers towhich a crosslinking agent has been applied. Generally, any cellulosicfiber that can be formed into a fibrous web and crosslinked is suitable.Similarly, suitable crosslinking agents include any agent thatcrosslinks cellulose at elevated temperature. The crosslinkable productcan be prepared by applying a crosslinking agent to a cellulosic fibrousweb and then drying the treated web without curing the crosslinkingagent. The resulting product is a dried fibrous web, preferably in anextended sheet form, that can be rolled, transported, and stored untiluse.

The crosslinkable fibrous product can be converted into a crosslinkedfibrous product by subjecting the product to conditions sufficient toeffect intrafiber crosslinking by, for example, heating to a temperatureto cure the crosslinking agent. The fibrous web can be heated at aboutthe crosslinking agent's cure temperature to provide a web that includescrosslinked fibers. Alternatively, the web can be fiberized, forexample, at a manufacturing site remote from initial web formation, andthe resulting individual crosslinkable fibers can be combined with otherfibers (e.g., hemp, bagasse, cotton, groundwood, bleached and unbleachedpulp, recycled or secondary fibers) and/or other materials (e.g.,superabsorbent particles and wet strength agents) to provide a fibrousweb containing crosslinkable cellulosic fibers. Subjecting such a web tocrosslinking conditions (e.g., heating at about the cure temperature)provides a fibrous web that includes, in addition to other fibers ormaterials, crosslinked cellulosic fibers. Thus, the crosslinkableproduct of the invention allows for the formation of crosslinked fibrousproducts at a site remote from crosslinkable product formation withouthaving to apply crosslinking agent at the site of ultimate webformation. The crosslinkable product further offers the advantage ofconvenience associated with transport to and use at the remote site byvirtue of its form as a rolled product.

Cellulosic fibers are a principal component of the crosslinkable fibrousproduct of this invention. Suitable fibers for forming the product ofthe invention are known to those skilled in the art and include anyfiber or fibrous mixture that can be crosslinked and from which afibrous web or sheet can be formed.

Although available from other sources, cellulosic fibers are derivedprimarily from wood pulp. Suitable wood pulp fibers for use with theinvention can be obtained from well-known chemical processes such as thekraft and sulfite processes, with or without subsequent bleaching. Pulpfibers can also be processed by thermomechanical, chemithermomechanicalmethods, or combinations thereof The preferred pulp fiber is produced bychemical methods. Groundwood fibers, recycled or secondary wood pulpfibers, and bleached and unbleached wood pulp fibers can be used.Softwoods and hardwoods can be used. Details of the selection of woodpulp fibers are well known to those skilled in the art. These fibers arecommercially available from a number of companies, includingWeyerhaeuser Company, the assignee of the present invention. Forexample, suitable cellulose fibers produced from southern pine that areusable with the present invention are available from WeyerhaeuserCompany under the designations CF416, NF405, PL416, FR516, and NB416.

The wood pulp fibers useful in the present invention can also bepretreated prior to use. This pretreatment may include physicaltreatment, such as subjecting the fibers to steam, or chemicaltreatment.

Although not to be construed as a limitation, examples of pretreatingfibers include the application of surfactants or other liquids, whichmodify the surface chemistry of the fibers. Other pretreatments includeincorporation of antimicrobials, pigments, dyes and densification orsoftening agents. Fibers pretreated with other chemicals, such asthermoplastic and thermosetting resins also may be used. Combinations ofpretreatments also may be employed. Similar treatments can also beapplied after formation of the fibrous product in post-treatmentprocesses.

Cellulosic fibers treated with particle binders and/ordensification/softness aids known in the art can also be employed inaccordance with the present invention. The particle binders serve toattach other materials, such as superabsorbent polymers, as well asothers, to the cellulosic fibers. Cellulosic fibers treated withsuitable particle binders and/or densification/softness aids and theprocess for combining them with cellulose fibers are disclosed in thefollowing U.S. patents: (1) U.S. Pat. No. 5,543,215, entitled “PolymericBinders for Binding Particles to Fibers”; (2) U.S. Pat. No. 5,538,783,entitled “Non-Polymeric Organic Binders for Binding Particles toFibers”; (3) U.S. Pat. No. 5,300,192, entitled “Wet Laid Fiber SheetManufacturing With Reactivatable Binders for Binding Particles toBinders”; (4) U.S. Pat. No. 5,352,480, entitled “Method for BindingParticles to Fibers Using Reactivatable Binders”; (5) U.S. Pat. No.5,308,896, entitled “Particle Binders for High-Bulk Fibers”; (6) U.S.Pat. No. 5,589,256, entitled “Particle Binders that Enhance FiberDensification”; (7) U.S. Pat. No. 5,672,418, entitled “ParticleBinders”; (8) U.S. Pat. No. 5,607,759, entitled “Particle Binding toFibers”; (9) U.S. Pat. No. 5,693,411, entitled “Binders for BindingWater Soluble Particles to Fibers”; (10) U.S. Pat. No. 5,547,745,entitled “Particle Binders”; (11) U.S. Pat. No. 5,641,561, entitled“Particle Binding to Fibers”; (12) U.S. Pat. No. 5,308,896, entitled“Particle Binders for High-Bulk Fibers”; (13) U.S. Pat. No. 5,498,478,entitled “Polyethylene Glycol as a Binder Material for Fibers”; (14)U.S. Pat. No. 5,609,727, entitled “Fibrous Product for BindingParticles”; (15) U.S. Pat. No. 5,571,618, entitled “ReactivatableBinders for Binding Particles to Fibers”; (16) U.S. Pat. No. 5,447,977,entitled “Particle Binders for High Bulk Fibers”; (17) U.S. Pat. No.5,614, 570, entitled “Absorbent Articles Containing Binder Carrying HighBulk Fibers; (18) U.S. Pat. No. 5,789,326, entitled “Binder TreatedFibers”; and (19) U.S. Pat. No. 5,611,885, entitled “Particle Binders”,each expressly incorporated herein by reference.

In addition to natural fibers, synthetic fibers including polymericfibers, such as polyolefin, polyamide, polyester, polyvinyl alcohol,polyvinyl acetate fibers, can also be incorporated into thecrosslinkable fibrous product. Suitable synthetic fibers include, forexample, polyethylene terephthalate, polyethylene, polypropylene, nylon,and rayon fibers. Other suitable synthetic fibers include those madefrom thermoplastic polymers, cellulosic and other fibers coated withthermoplastic polymers, and multicomponent fibers in which at least oneof the components includes a thermoplastic polymer. Single andmulticomponent fibers can be manufactured from polyester, polyethylene,polypropylene, and other conventional thermoplastic fibrous materials.Single and multicomponent fibers are commercially available. Suitablebicomponent fibers include Celbond® fibers available fromHoechst-Celanese Company. The fibrous product can also includecombinations of natural and synthetic fibers.

The fibrous product of the present invention includes a crosslinkingagent. The crosslinking agent useful in the present invention is alatent crosslinking agent in that, although the crosslinking agent isapplied to a web of cellulose fibers and the resulting treated fibersdried to provide the crosslinkable product, no significant crosslinkingoccurs during the product's formation process and the resulting productis substantially free from crosslinks, particularly interfibercrosslinks. This enables the sheet to be readily defibered, e.g., in ahammermill, without excessive energy requirements and the production ofdust and knots normally associated with defibration of crosslinked webs.Suitable crosslinking agents useful in the invention do not crosslinkcellulose to any significant degree under the conditions for forming thecrosslinkable product.

Crosslinking typically requires heating treated fibers at a temperatureand for a time sufficient to cure the crosslinking agent. By avoidingsubjecting treated cellulosic fibers to elevated temperature,crosslinking can be prevented. Thus, in one embodiment of the presentinvention, suitable crosslinking agents include those that do notundergo crosslinking at temperatures below the highest temperature(i.e., drying temperature) of the product's formation process (i.e., theformation process does not reach the cure temperature of thecrosslinking agent).

Alternatively, because the crosslinking reaction is typically pHdependent, the crosslinking agent can be applied to the cellulosicfibers at a pH at which no crosslinking can occur. Generally,crosslinking optimally occurs in the pH range from about 2 to about 4.Application of the crosslinking agent at a pH outside of thecrosslinking pH range will substantially prevent crosslinking. As withthe optimal pH for crosslinking, the actual pH for preventing thecrosslinking reaction will depend on the particular crosslinking agentand the formation conditions employed.

For crosslinking agents that undergo crosslinking in the presence of acatalyst, application of the crosslinking agent without the catalyst canalso provide a crosslinkable product that is substantially free fromcrosslinks.

Suitable cellulose crosslinking agents include crosslinking agents knownin the art such as aldehyde and urea-based formaldehyde additionproducts. See, for example, U.S. Pat. Nos. 3,224,926; 3,241,533;3,932,209; 4,035,147; 3,756,913; 4,689,118; 4,822,453; U.S. Pat. No.3,440,135, issued to Chang; U.S. Pat. No. 4,935,022, issued to Lash etal.; U.S. Pat. No. 4,889,595, issued to Herron et al.; U.S. Pat. No.3,819,470, issued to Shaw et al.; U.S. Pat. No. 3,658,613, issued toSteiger et al.; and U.S. Pat. No. 4,853,086, issued to Graef et al., allof which are expressly incorporated herein by reference in theirentirety. Other suitable crosslinking agents include carboxylic acidcrosslinking agents such as polycarboxylic acids. U.S. Pat. Nos.5,137,537; 5,183,707; and 5,190,563, describe the use of C₂-C₉polycarboxylic acids that contain at least three carboxyl groups (e.g.,citric acid and oxydisuccinic acid) as crosslinking agents.

Suitable urea-based crosslinking agents include substituted ureas suchas methylolated ureas, methylolated cyclic ureas, methylolated loweralkyl cyclic ureas, methylolated dihydroxy cyclic ureas, dihydroxycyclic ureas, and lower alkyl substituted cyclic ureas. Specificpreferred urea-based crosslinking agents include dimethyldihydroxy urea(DMDHU, 1,3-dimethyl-4,5-dihydroxy-2-imidazolidinone),dimethyloldihydroxyethylene urea (DMDHEU,1,3-dihydroxymethyl-4,5-dihydroxy-2-imidazolidinone), dimethylol urea(DMU, bis[N-hydroxymethyl]urea), dihydroxyethylene urea (DHEU,4,5-dihydroxy-2-imidazolidinone), dimethylolethylene urea (DMEU,1,3-dihydroxymethyl-2-imidazolidinone), and dimethyldihydroxyethyleneurea (DDI, 4,5-dihydroxy-1,3-dimethyl-2-imidazolidinone). Other suitablesubstituted ureas include glyoxal adducts of ureas.

Suitable polycarboxylic acid crosslinking agents include citric acid,tartaric acid, malic acid, succinic acid, glutaric acid, citraconicacid, itaconic acid, tartrate monosuccinic acid, and maleic acid. Otherpolycarboxylic acid crosslinking agents include polymeric polycarboxylicacids such as poly(acrylic acid), poly(methacrylic acid), poly(maleicacid), poly(methylvinylether-co-maleate) copolymer,poly(methylvinylether-co-itaconate) copolymer, copolymers of acrylicacid, and copolymers of maleic acid. The use of polymeric polycarboxylicacid crosslinking agents such as polyacrylic acid polymers, polymaleicacid polymers, copolymers of acrylic acid, and copolymers of maleic acidis described in U.S. patent application Ser. No. 08/989,697, filed Dec.12, 1997, and assigned to Weyerhaeuser Company. Mixtures or blends ofcrosslinking agents can also be used.

In a preferred embodiment, the crosslinking agent is a substituted urea.

The crosslinking agent is present in the fibrous product in an amountfrom about 1 to about 10 percent by weight, and preferably from about 4to about 6 percent by weight, based on the total weight of the fibrousproduct.

In another aspect, the present invention provides a method for forming acrosslinkable cellulosic fibrous product. The crosslinkable product isformed by applying a crosslinking agent to a mat of cellulosic fibersand then drying the treated mat such that substantially no crosslinkingoccurs and the product is substantially free from crosslinks. Thecellulosic fibrous mat can be laid using conventional papermakingmethods and devices. Once laid, the fibrous mat is then treated with acrosslinking agent. The crosslinking agent can be applied as a solutionto one or both surfaces of the mat using any one of a variety of methodsknown in the art including, for example, spraying, rolling, and dipping.Cellulosic fibers in the treated mat are coated and/or impregnated withthe crosslinking agent. After the crosslinking agent has been applied,the treated mat is then dried without heating to a temperaturesufficient to cure the crosslinking agent.

The cellulosic web formed in accordance with the present invention canbe an extended sheet having sheet strength sufficient to permit thesheet to be formed into a rolled product. Thus, in one embodiment, thepresent invention provides a crosslinkable fibrous product in the formof a rolled good that is readily transported, stored, and used.

The product of the invention can be a source for crosslinked cellulosicfibers and, as such, can be used as a raw material for processes thatincorporate crosslinked fibers into a variety of fibrous products. Thus,in a further aspect, the present invention provides a method for usingthe crosslinkable product described above.

In one method, the crosslinkable product is separated intoindividualized fibers, at least a portion of which are coated and/orimpregnated with a crosslinking agent. The individualized fibers arethen incorporated into a fibrous web and further processed by, forexample, combining with additional fibers or materials and/or subjectingthe fibrous web to further chemical treatments. Ultimately, the fibrousweb that includes fibers from the crosslinkable product is thensubjected to conditions (e.g., application of heat, catalyst, or pHadjustment) sufficient to cure the crosslinking agent and to provide acrosslinked product.

The crosslinkable product of the present invention can be supplied in afibrous rolled form and readily incorporated into subsequent processes.For example, the crosslinkable product can be readily separated intoindividual fibers by fiberization.

The rolled form of the crosslinkable product can be continuouslyintroduced into a fiberizer to provide individualized fibers that arecoated/impregnated with crosslinking agent. These individualized fiberscan be formed into a fibrous web by, for example, air or wet laying thefibers onto a foraminous support (e.g., forming wire). Theseindividualized fibers can be combined with additional fibers (e.g.,other cellulosic fibers such as wood pulp fibers and/or syntheticfibers) prior to or after laying. The fibers can also be combined withother materials including wet-strength agents, particle binders, orparticles such as absorbent particles (e.g., superabsorbent polymers).

Once deposited, the crosslinkable fibers can be further chemicallytreated to, for example, effect crosslinking. For example, the fibrousweb can be heated at a temperature and for a time sufficient to cure thecrosslinking agent and to provide a crosslinked fibrous product. Anothermethod for effecting crosslinking in a web that includes fibers from thecrosslinkable product is to treat the fibrous web with a crosslinkingcatalyst and then heating the resulting web to cure the crosslinkingagent. Suitable crosslinking catalysts, known to those in the art,accelerate the bonding reaction between the crosslinking agent andcellulose fiber. Catalysts can include acidic salts, such as ammoniumchloride, ammonium sulfate, aluminum chloride, magnesium chloride,magnesium nitrate, and alkali metal salts of phosphorous-containingacids. In a preferred embodiment, the crosslinking catalyst is sodiumhypophosphite.

Another method for crosslinking a web that includes fibers from thecrosslinkable product involves adjusting the pH of the web to facilitatethe crosslinking reaction. For example, a solution can be applied to wetthe web and to bring the pH into a range such that the crosslinkingreaction will occur on heating (e.g., adjust to about pH 2-4 withaqueous acid).

Other chemical treatments include those known in pulp and papermakingmethods. For example, the hydrophilicity of the fibers or the tensilestrength of the web can be modified by chemical treatment. To increasethe strength of the resulting crosslinked fibrous web, a wet-strengthagent can be applied to the web, preferably prior to curing andcrosslinking. Suitable wet-strength agents include cationic modifiedstarch having nitrogen-containing groups (e.g., amino groups) such asthose available from National Starch and Chemical Corp., Bridgewater,N.J.; latex; wet-strength resins such as polyamide-epichlorohydrin resin(e.g., Kymene® 557LX, Hercules, Inc., Wilmington, Del.), polyacrylamideresin (described, for example, in U.S. Pat. No. 3,556,932 issued Jan.19, 1971 to Coscia et al.; also, for example, the commercially availablepolyacrylamide marketed by American Cyanamid Co., Stanford, Conn., underthe trade name Parez™ 631 NC); urea formaldehyde and melamineformaldehyde resins, and polyethylenimine resins. A general discussionon wet-strength resins utilized in the paper field, and generallyapplicable in the present invention, can be found in TAPPI monographseries No. 29, “Wet Strength in Paper and Paperboard”, TechnicalAssociation of the Pulp and Paper Industry (New York, 1965).

The crosslinkable cellulosic fibrous product of the present inventioncan be utilized to prepare crosslinked cellulosic fibrous products thatcan be advantageously incorporated into a variety of absorbent articles,such as diapers, including disposable diapers and training pants;feminine care products, including sanitary napkins, tampons, and pantliners; adult incontinence products; toweling; surgical and dentalsponges; bandages; food tray pads; and the like.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A crosslinkable fibrousproduct comprising cellulosic fibers and a crosslinking agent, whereinthe product comprises substantially noncrosslinked fibers, and whereinthe product is a rolled sheet.
 2. The product of claim 1 wherein thecrosslinking agent is present in an amount from about 1 to about 10 centby weight based on the total weight of the product.
 3. The product ofclaim 1 wherein the crosslinking agent is present in an amount fromabout 4 to about 6 percent by weight based on the total weight of theproduct.
 4. The product of claim 1 wherein the crosslinking agent isselected from the group consisting of substituted ureas, polycafboxylicacids, and glyoxal adducts of ureas.
 5. The product of claim 1 furthercomprising a particle binder.
 6. The product of claim 1 furthercomprising a wet-strength agent.
 7. The product of claim 1 wherein thewet-strength agent comprises a polyamide-epichlorohydrin resin.
 8. Theproduct of claim 1 wherein the wet-strength agent comprises apolyacrylamide resin.
 9. The product of claim 1 wherein the wet-strengthagent comprises a latex.
 10. A crosslinkable fibrous product comprisingcellulosic fibers treated with a crosslinking agent and a wet-strengthagent, wherein the product comprises substantially noncrosslinkedfibers, and wherein the product is a rolled sheet.